1. Field of the Invention
This invention relates to continuously-variable monolithic radio-frequency (RF) and microwave analog delay lines and their use in analog devices employing these analog delay lines, such as direction finders and transversal filters by way of examples.
2. Description of the Prior Art
Digital signal-processing techniques are well known in the art, and are employed, inter alia, for implementing delay-line circuits (e.g. shift registers), transversal filters, Fast Fourier transforms (FFT), autocorrelation analyses, etc. Up to medium radio frequencies, it is feasible to sample and digitize signals at precisely spaced times. However, at high radio and microwave frequencies, digitizing analog input signals is not yet practical. Therefore, equivalent analog signal-processing techniques may be performed at high radio and microwave frequencies using sequential time delay elements in conjunction with suitable microwave combinational elements or circuits. However, it should be understood that while from a physical point of view the signals processed by such sequential time delay elements are analog signals, from an informational point of view the signals may be comprised of a sequence of digitally (e.g., binary) coded pulses that are not precisely spaced rather than being comprised of continuous signals. Examples of known time delay elements for implementing such analog signal-processing include surface-acoustic-wave (SAW) devices and acoustic-charge-transport (ACT) devices.
Also known in the microwave and radio frequency analog art is a dual-gate field-effect transistor (FET). An example of such a dual-gate (FET) is a dual-gate GaAs MESFET, manufactured by NEC and designated NE 25139, which heretofore has been used as an RF amplifier and as a mixer in UHF applications.
Further known is that carriers in a semiconductor material subject to an applied electric field drift at a drift velocity that is a function of both the particular type of semiconductor material and the strength of the applied electric field.